Mike,
If you catch one of the High Speed trains that will stop at Westenhanger at certain times, it get you to St Pancras in around 45 mins, from there you get an Underground train straight through to Heathrow terminals, could not be easier. If there is not a High Speed stopping at Westenhanger when you wish to go, just get a local train to Ashford International and catch one there. This is how I get to Heathrow.